APPLICATION HINTS
External Capacitors
Since the AMS2951’s dropout voltage is load dependent (see
curve in typical performance characteristics), the input voltage trip
point (about 5V) will vary with the load current. The output
voltage trip point (approx. 4.75V) does not vary with load.
The error comparator has an open-collector output which requires
an external pull-up resistor. This resistor may be returned to the
output or some other supply voltage depending on system
requirements. In determining a value for this resistor, note that the
output is rated to sink 400µA, this sink current adds to battery
drain in a low battery condition. Suggested values range from
100K to 1MΩ. The resistor is not required if this output is unused.
A 1.0 µF or greater capacitor is required between output and
ground for stability at output voltages of 5V or more. At lower
output voltages, more capacitance is required (2.2µ or more is
recommended for 3V and 3.3V versions). Without this capacitor
the part will oscillate. Most types of tantalum or aluminum
electrolytic works fine here; even film types work but are not
recommended for reasons of cost. Many aluminum types have
electrolytes that freeze at about -30°C, so solid tantalums are
recommended for operation below -25°C. The important
parameters of the capacitor are an ESR of about 5 Ω or less and
resonant frequency above 500 kHz parameters in the value of the
capacitor. The value of this capacitor may be increased without
limit.
Programming the Output Voltage (AMS2951)
The AMS2951 may be pin-strapped for the nominal fixed output
voltage using its internal voltage divider by tying the output and
sense pins together, and also tying the feedback and VTAP pins
together. Alternatively, it may be programmed for any output
voltage between its 1.235V reference and its 30V maximum
rating. As seen in Figure 2, an external pair of resistors is
required.
At lower values of output current, less output capacitance is
required for stability. The capacitor can be reduced to 0.33 µF for
currents below 10 mA or 0.1 µF for currents below 1 mA. Using
the adjustable versions at voltages below 5V runs the error
amplifier at lower gains so that more output capacitance is needed.
For the worst-case situation of a 200mA load at 1.23V output
(Output shorted to Feedback) a 3.3µF (or greater) capacitor should
be used.
The complete equation for the output voltage is:
Unlike many other regulators, the AMS2950, will remain stable
and in regulation with no load in addition to the internal voltage
divider. This is especially important in CMOS RAM keep-alive
applications. When setting the output voltage of the AMS2951
version with external resistors, a minimum load of 1µA is
recommended.
A 1µF tantalum or aluminum electrolytic capacitor should be
placed from the AMS2950/AMS2951 input to the ground if there
is more than 10 inches of wire between the input and the AC filter
capacitor or if a battery is used as the input.
Stray capacitance to the AMS2951 Feedback terminal can cause
instability. This may especially be a problem when using a higher
value of external resistors to set the output voltage. Adding a 100
pF capacitor between Output and Feedback and increasing the
output capacitor to at least 3.3 µF will fix this problem.
Vout = VREF × (1 + R1/ R2)+ IFBR1
where VREF is the nominal 1.235 reference voltage and IFB is the
feedback pin bias current, nominally -20 nA. The minimum
recommended load current of 1 µA forces an upper limit of 1.2
MΩ on value of R2, if the regulator must work with no load (a
condition often found in CMOS in standby) IFB will produce a 2%
typical error in VOUT which may be eliminated at room
temperature by trimming R1. For better accuracy, choosing R2 =
100k reduces this error to 0.17% while increasing the resistor
program current by 12 µA. Since the AMS2951 typically draws 60
µA at no load with Pin 2 open-circuited, this is a small price to
pay.
Reducing Output Noise
Error Detection Comparator Output
In reference applications it may be an advantageous to reduce the
AC noise present at the output. One method is to reduce the
regulator bandwidth by increasing the size of the output capacitor.
This is the only way that noise can be reduced on the 3 lead
AMS2950 but is relatively inefficient, as increasing the capacitor
from 1 µF to 220 µF only decreases the noise from 430 µV to 160
µV rms for a 100 kHz bandwidth at 5V output.
The comparator produces a logic low output whenever the
AMS2951 output falls out of regulation by more than
approximately 5%. This figure is the comparator’s built-in offset
of about 60 mV divided by the 1.235 reference voltage (Refer to
the block diagram). This trip level remains “5% below normal”
regardless of the programmed output voltage of the 2951. For
example, the error flag trip level is typically 4.75V for a 5V output
or 11.4V for a 12V output. The out of regulation condition may be
due either to low input voltage, current limiting, or thermal
limiting.
Noise could also be reduced fourfold by a bypass capacitor across
R1, since it reduces the high frequency gain from 4 to unity. Pick
CBYPASS ≅ 1 / 2πR1 × 200 Hz
Figure 1 gives a timing diagram depicting the ERROR signal and
the regulator output voltage as the AMS2951 input is ramped up
and down. For 5V versions the ERROR signal becomes valid
(low) at about 1.3V input. It goes high at about 5V input (the input
voltage at which Vout = 4.75 ).
or about 0.01 µF. When doing this, the output capacitor must be
increased to 3.3 µF to maintain stability. These changes reduce
the output noise from 430 µV to 100 µV rms for a 100 kHz
bandwidth at 5V output. With the bypass capacitor added, noise
no longer scales with output voltage so that improvements are
more dramatic at higher output voltages.
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